This invention relates to a method of injecting chemicals into a hydrocarbon reservoir for the purpose of increasing hydrocarbon recovery. More particularly, the invention pertains to the use of a sacrificial agent system of lignosulfonate and starch, or lignosulfonate and cellulose to decrease adsorption of surfactants within hydrocarbon reservoirs.
One of the most vexing problems in the use of surfactant flooding for enhanced oil recovery is the frequent, substantial loss of surfactant due to adsorption on the formation matrix and precipitation by polyvalent cations such as calcium and magnesium. A significant percentage of surfactants are also physically entrapped within the pore spaces of the rock matrix. Of chief concern is surfactant adsorption on the formation matrix which significantly decreases surfactant flood efficiency, making it necessary to inject a greater quantity of surfactant and increasing the cost of any surfactant flood.
Additionally, most surfactants are satisfactory for surfactant flooding only if the calcium and magnesium concentrations of the formation water fall below about 500 ppm. Petroleum sulfonates, the most popular type of surfactants, precipitate where divalent ion concentrations exceed about 500 ppm. Such precipitation renders the sulfonates inoperative for recovering oil and in some instances, causes formation plugging.
The most promising approach for reducing the amount of surfactants retained by the formation matrix has been to use sacrificial agent compounds, either in a preflush solution injected before the surfactant-containing solution, or in the surfactant solution. The compounds are sacrificial in that their adsorption on the formation matrix and entrapment within the pore spaces of the formation reduces the loss of the more expensive surfactants, solubilizers and polymers contained within the surfactant solutions.
Various chemicals have been employed as sacrificial agents to decrease the adsorption of surfactants or to tie up polyvalent cations and prevent them from precipitating surfactants from the flooding medium. Lignosulfonates form one class of compounds which have been found to have excellent properties as sacrificial agents. They are economically attractive because they are by-products of the pulp industry. Supply is plentiful and product costs are much less than the costs of surfactants employed in enhanced oil recovery floods. The use of various lignosulfonates has been disclosed extensively in the literature.
U.S. Pat. Nos. 4,157,115 and 4,271,906 disclose several methods of using aqueous solutions of lignosulfonate salts as sacrificial agents. The use of oxidized lignosulfonates is disclosed in U.S. Pat. Nos. 4,133,385 and 4,196,777. Chrome lignosulfonates as sacrificial agents are described in U.S. Pat. No. 4,142,582. U.S. Pat. No. 4,172,497 discloses the use of lignosulfonates carboxylated with chloroacetic acid and U.S. Pat. No. 4,172,498 discloses sulfomethylated lignosulfonates as sacrificial agents. U.S. Pat. No. 4,479,542 describes a sacrificial afterflush method employing lignosulfonates.